Simple implementation of the RC4 stream cipher in Swift. Currently only encrypts. Feel free to make it better.

RC4.swift

//
//  RC4
//
//  Created by Bradley Slayter on 2/2/16.
//  Copyright © 2016 Bradley Slayter. All rights reserved.
//


import Foundation

struct ByteStream: GeneratorType {
    typealias Element = UInt8
    
    var i = 0
    var j = 0
    var key: [UInt8]
    
    init(key: [UInt8]) {
        self.key = key
    }
    
    mutating func next() -> Element? {
        i = (i + 1) % 256
        j = (j + Int(key[i])) % 256
        (key[i], key[j]) = (key[j], key[i])
        return key[(Int(key[i]) + Int(key[j])) % 256]
    }
}

func keyInit(key: [UInt8]) -> [UInt8] {
    var K = [UInt8]()
    for i in 0..<256 {
        K.append(UInt8(i))
    }
    
    var j = 0
    for i in 0..<256 {
        let index = Int(K[i])
        j = (j + index + Int(key[i % key.count])) % 256
        (K[i], K[j]) = (K[j], K[i])
    }
    
    return K
}

func encrypt(text: [UInt8], key: [UInt8]) -> String {
    var cipherText = [UInt8]()
    var byteStream = ByteStream(key: key)
    
    for ch in text {
        let cipherByte = ch ^ byteStream.next()!
        cipherText.append(cipherByte)
    }
    
    return String(bytes: cipherText, encoding: NSASCIIStringEncoding)!
}

print("Enter a key> ", separator: "", terminator: "")
let key = readLine(stripNewline: true)

print("Enter a message> ", separator: "", terminator: "")
let message = readLine(stripNewline: true)

let keyStream = keyInit(Array(key!.utf8))
let cipherText = encrypt(Array(message!.utf8), key: keyStream)
print("Cipher text: \(cipherText)")